Plants to Plastic: Polymerization of PA-11, a renewable plastic

I remember being about seven years old and asking my mom, after staring at a plastic straw for a few minutes, “Where does plastic come from?” Her response was the typical answer — plastic is made out of oil. As a child, I thought this was weird, because I knew that petroleum oil was an icky-sticky gross black liquid crud that cars needed for some reason and I couldn’t comprehend how it made the straw for a juice box, but I accepted the answer anyway.

So that was the answer I accepted for a pretty long time. Plastic came from a non-renewable resource, which, is pretty sad stuff when you consider all the Tupperware or tossed plastic bottles in the world. However, much to the presumed surprise of seven year old me, there are other sources of plastics; plants.

This answer would have been just as weird to me as a kid, but makes much more sense now. Polymers are made of monomer units, and most plastics are made from petrochemicals. But, polyamide-11 (PA-11), the polymer I work with most often in lab, is not derived from petrochemicals. Its made from the mouthful-monomer 11-Aminoundecanoic acid which is made from an often infamous plant, the castor oil plant.

Most people associate the castor bean plant with two unpleasant things — a home remedy castor oil and ricin. This plant has more to offer than unwanted medicine and umbrella assassinations, though. This plant is the source of PA-11 monomer, which allows PA-11 to be produced as a biopolymer from renewable resources. In Dr. Kranbuehl’s, we have done several polymerizations of PA-11 monomer.

The monomer starts out like many things in chemistry — a rather non-descript white powder. The amount we want to polymerize is weighed out into a beaker and then spread out, as smooth as possible, into a teflon-lined baking dish. Like many things in lab, it is very important that this remain anaerobic during the polymerization — if oxygen is present, the polymer will come out of the oven as an oxidized brown/black piece of plastic. To prevent this, the dish of monomer is gently blown with argon before being placed in the oven. Because argon is heavier than the air, it sits in a layer in the dish. When the dish is safely in the oven, argon flows through it during the polymerization to prevent oxidation. Quite a few polymerizations have been performed this summer and include:

PA-11 Monomer polymerized for 4 hours at 220C

PA-11 Monomer polymerized for 3 hours at 230C

PA-11 Monomer polymerized for 4 hours at 225C

Here are some examples of polymerized PA-11 done in-house:

The dish is lined with Teflon. This sample is not very oxidized at all — it is still very white.

A very oxidized sample of polymer

This sample has varying degrees of oxidation, more oxidation where it is darker, and less where the color is lighter

Beyond 240C, the PA-11 will become cross-linked, which is not what we are aiming for in anyway. Cross-linkages make the molecular weight difficult to determine, and mean the polymer cannot be solvated in m-cresol or HFIP for CIV runs or SEC-MALLS runs, respectively.

In short, the polymerization of this bioplastic in lab is a one-ingredient baking process; it reminds me of making baking with a single ingredient, at least. Once we have a good, non-oxidized, freshly polymerized sheet of bio polymer, it can be punched into standard shapes and go onto the normal runs of tests and be put into an ageing study.

Punching a sheet of in-house polymerized polymer with an industrial-level hole punch

Punched Samples

The polymerization of PA-11 is interesting from an environmental standpoint because of the renewable castor oil plant. PA-11’s close cousin, PA-12, is not a biopolymer and the production of PA-11 requires more non-renewable resources. PA-11 on the other hand is produced with entirely renewable resources and maintains similar high performance properties to PA-12.

Our in-polymerized PA-11 samples are not commercially produced but there are commercial PA-11 products. The company Arkema sells PA-11 and PA-12 under the name Rilsan. Although one might not encounter PA-11 or PA-12 everyday, it is used in fuel lines of cars. Unlike polyethylene, PA-11/PA-12 won’t dissolve in alkanes, which make up gasoline (largely octane) so they’re a great choice for high performance polymer fuel lines. They are also used in oil pipe lines, cable sheathing, and in some electronic devices. Because of these large and far reaching uses of these polyamides, the use of a renewable source to produce PA-11 is of great interest.